Mobile terminals such as cellular phones have become ubiquitous in modern society. Mobile terminals rely on sending an electromagnetic signal through the air to a base station and receiving electromagnetic signals through the air from the base station. An unfortunate side effect of the convenience of this wireless communication is that the signal-carrying electromagnetic radiation that forms the backbone of the communication may interfere with other electronic devices. This phenomenon is known as electromagnetic interference (EMI) or electromagnetic compatibility (EMC).
While interfering with other electronic devices like a computer or television is problematic, it is also possible for multiple mobile terminals operating in proximity to one another to have cross channel EMI. That is, one mobile terminal may be transmitting in a first channel, but some of the signal may spill over as noise into channels that are nearby in the frequency spectrum and on which a second mobile terminal is trying to operate. This spill over transmission is known by various terms, but is termed herein as “side band transmission.”
To combat EMI in the United States, the FCC has promulgated standards for emissions that limit how much radiation may be radiated within certain frequency bands. On top of the FCC emissions rules, the various communication protocols used by mobile terminals may impose more restrictive limitations with specific attention paid to side band transmission levels. For example, Annex A of the GSM 05.05 version 8.5.1, released 1999, indicates that the maximum allowed signal for spurious side band signals is the larger of −60 dBc or −36 dBm. This measurement is to be averaged over at least two hundred transmit power cycles.
Against the backdrop of these standards, many mobile terminals incorporate DC—DC converters in their internal circuitry to change a DC voltage level of a battery to a lower or higher DC voltage level depending on the needs of the internal circuitry of the mobile terminal. A common method to implement a DC—DC converter uses a switching power supply that has a switch that opens and closes at a predetermined frequency according to a clock signal. Such switching power supplies exhibit a periodic ripple in their output at the switching frequency. If the DC—DC converter is used to provide a supply voltage (Vcc) to a saturated power amplifier, this ripple may mix with the radio frequency carrier to generate spurious side band signals.
To combat this ripple, manufacturers tend to use low drop-out linear regulators for power control associated with power amplifiers instead of the switching DC—DC converters. This substitution avoids the ripple issues, but does so at the expense of decreased efficiency and shorter battery life. Thus, there exists a need for a way to reduce spurs in a power amplifier's output while using an efficient switching power supply as a supply voltage for power amplifiers.